In recent years, a secondary battery represented by a lithium-ion battery is attached not only to a portable device such as a mobile phone or a digital camera but also to an electric bicycle, an electric motorcycle, or an automobile. With the diversification of the applications of the secondary battery, battery weight reduction and increased design freedom have become current requirements. A film-sheathed battery that uses a light and flexible film sheath material has emerged as a secondary battery that is capable of satisfying these requirements. In addition, for applications that require a large amount of power, there are battery packs that are configured by laminating a plurality of film-sheathed batteries.
Film-sheathed batteries generally include a power generating element provided with a positive electrode and a negative electrode, and a film sheath material to cover the power generating element. The film sheath material preferably serves to prevent electrolyte injected therein from leaking or moisture from entering into the battery. Therefore, on the outer peripheral part of the film sheath material, a sealing part in which resin surfaces are thermally bonded to each other is provided. The sealing part is a portion that does not function as a battery. Accordingly, when the space that a sealing part occupies in an installation space for a battery is large, volumetric efficiency (energy efficiency per unit volume) deteriorates. Thus, a technology for solving such a problem is disclosed in Patent document 1.
FIG. 1 is a perspective view illustrating a sheet type battery disclosed in Patent document 1. In the sheet type battery illustrated in FIG. 1, unit cell 100 is housed in two thermal bonding seal films 150. Unit cell 100 includes a positive electrode that is provided with positive electrode layer 160 and current collector 170, a negative electrode that is provided with negative electrode layer 120 and current collector 130, and solid electrolyte layer 180 that is formed between the positive electrode and the negative electrode. Positive electrode terminal 90 is fitted to the positive electrode. Negative electrode terminal 110 is fitted to the negative electrode.
Thermal bonding part 140 is provided on the outer peripheral part of thermal bonding seal film 150. Thermal bonding part 140 is folded along the shape of unit cell 100 to be bonded to the upper surface of thermal bonding seal film 150. Therefore, the outer dimensions of the sheet type battery are reduced, and volumetric efficiency can be improved.